The epididymis reabsorbs 95 percent of the fluid leaving the testis. In the process it secretes and reabsorbs a variety of inorganic and organic molecules. By utilizing micropuncture techniquus we plan to more clearly define some of the mechanisms involved in this transport in rats. Initially, we will sample fluid from the initial segment and the intermediate zone, the regions in which the bulk of epididymal transport occurs. The fluid will be analyzed for Na ion, K ion and Cl minus concentrations as well as for osmolality and pH. This will enable us to complete the detailed investigation of the composition of fluid in the different regions of the epididymis, as well as to ascertain the specific sites of acidification and weak electrolyte secretion. We will then microperfuse, "in vivo", portions of the different regions of the epididymis with solutions of known composition. This will allow us to calculate precisely the rates of fluid and electrolyte reabsorption as well as the rate of organic molecule secretion. Microperfusion will also enable us to precisely determine the different characteristics of transport in these regions. Using suitable animal preparations hormonal regulation of epididymal transport in the various regions will be investigated. In addition, the effects of pharmacological agents on epidymal transport will be determined. Finally, we will attempt to develop an "in vitro" perfusion system for various segments of the epididymis. This system will be used to investigate organic molecule secretion, its coupling to electrolyte transport, as well as the hormonal control of this transport.